missing @font-face in github-markdown.css

The new generated 'github-markdown.css' missed `@font-face `. Therefore it cannot compiled by sass or other css generator.

https://github.com/sindresorhus/github-markdown-css/commit/0d25bb4525eab80cbe95a2d18977a2ad91a7299e#diff-f97b37ced0d72ea4faa45855d1aeb078